feat: scaffold CalendulaApp + MainActivity with themed placeholder

CalendulaApp registers the Hilt component graph. MainActivity uses
enableEdgeToEdge() and renders a centered placeholder showing app
name + tagline via CalendulaTheme. PlaceholderPreview lets the IDE
preview the layout. Resolves the pre-existing MissingClass lint
error from the manifest reference.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/src/main/java/de/jeanlucmakiola/calendula/CalendulaApp.kt

@@ -0,0 +1,11 @@
+package de.jeanlucmakiola.calendula
+
+import android.app.Application
+import dagger.hilt.android.HiltAndroidApp
+
+/**
+ * Application entry point. Registered as android:name=".CalendulaApp"
+ * in AndroidManifest.xml. Hilt initializes its component graph here.
+ */
+@HiltAndroidApp
+class CalendulaApp : Application()

app/src/main/java/de/jeanlucmakiola/calendula/MainActivity.kt

@@ -0,0 +1,59 @@
+package de.jeanlucmakiola.calendula
+
+import android.os.Bundle
+import androidx.activity.ComponentActivity
+import androidx.activity.compose.setContent
+import androidx.activity.enableEdgeToEdge
+import androidx.compose.foundation.layout.Arrangement
+import androidx.compose.foundation.layout.Column
+import androidx.compose.foundation.layout.fillMaxSize
+import androidx.compose.foundation.layout.padding
+import androidx.compose.material3.MaterialTheme
+import androidx.compose.material3.Scaffold
+import androidx.compose.material3.Text
+import androidx.compose.runtime.Composable
+import androidx.compose.ui.Alignment
+import androidx.compose.ui.Modifier
+import androidx.compose.ui.res.stringResource
+import androidx.compose.ui.tooling.preview.Preview
+import dagger.hilt.android.AndroidEntryPoint
+import de.jeanlucmakiola.calendula.ui.theme.CalendulaTheme
+
+@AndroidEntryPoint
+class MainActivity : ComponentActivity() {
+    override fun onCreate(savedInstanceState: Bundle?) {
+        super.onCreate(savedInstanceState)
+        enableEdgeToEdge()
+        setContent {
+            CalendulaTheme {
+                Scaffold(modifier = Modifier.fillMaxSize()) { innerPadding ->
+                    PlaceholderScreen(modifier = Modifier.padding(innerPadding))
+                }
+            }
+        }
+    }
+}
+
+@Composable
+private fun PlaceholderScreen(modifier: Modifier = Modifier) {
+    Column(
+        modifier = modifier.fillMaxSize(),
+        verticalArrangement = Arrangement.Center,
+        horizontalAlignment = Alignment.CenterHorizontally,
+    ) {
+        Text(
+            text = stringResource(R.string.app_name),
+            style = MaterialTheme.typography.displayMedium,
+        )
+        Text(
+            text = stringResource(R.string.app_tagline),
+            style = MaterialTheme.typography.bodyLarge,
+        )
+    }
+}
+
+@Preview(showBackground = true)
+@Composable
+private fun PlaceholderPreview() {
+    CalendulaTheme { PlaceholderScreen() }
+}